1. Field of the Invention
The invention relates to distributed communications systems, in which relatively short packets of data are transmitted by radio transmissions of very low power, and relatively long periods of time pass between transmissions. Such a communications system is proposed for use in a centralized system for controlling devices located in many places throughout the building. For example, the invention is applicable to centralized control of artificial lighting systems in buildings where each room or area has an individual local control, or to heating and/or air conditioning systems having many individually controllable heating devices, heat pumps (heating or cooling) or heat exchangers; to occupancy-sensing, security, or fire detection systems requiring sensed-data transmission from a multiplicity of sensor locations to a central location, as well as control signals transmitted back to selected locations.
For convenience, this specification refers extensively to a "building" or "building computer." It should be clear that the term "building" should be interpreted as including a portion of a building, or a building complex having two or more structures or portions thereof under common control, and sharing one network; and might be applicable to an amusement park or other outdoor situation.
Other applications not directed at energy conservation might include centralized override of volume or channel settings for an existing hard-wired background music and public address system, and in particular would allow control on a group or zone basis where this had not been provided in the hard-wiring layout.
2. Description of the Prior Art
The need to provide low data rate transmission to large numbers of locations has usually been met by hard-wired systems, because of the value of channels in the radio spectrum. For example, central building control of lighting or ventilating and air conditioning units has been possible for many years, through use of low voltage wiring from a central office or computer to operate relays controlling local power circuits. Such a system is very difficult to retrofit in an existing building. A further difficulty with this type of system is that correction of wiring failures is often difficult because wiring drawings are inaccurate or non-existent, and tracing these low voltage cables is time consuming and expensive.
A low-power radio control system for receivers up to 75 feet away is described in a paper "SONA/ECS, a decentralized environmental control system (for disabled)" published as part of the Proceeding of the IEEE computer Society Workshop on Computing to Aid the Handicapped, Nov. 4-5, 1982.
Another system for building control makes use of the existing AC power wiring in the building to carry control signals. So-called "carrier current" systems for impressing a relatively low radio frequency on AC power wiring have been used for telemetering data in power distribution systems, and for "wireless" intercom or music systems, but have been prone to excessive noise. To reduce cost, 900 MHz radio channels for 2-way communication are described in "900 MHz radio provides two-way path for control and return" is described in Transm. and Distribution, vol. 36, no. 6, pp 33-6 for June 1984. This system had the advantage that it was claimed to be installable and maintainable by the utility's own workforce.
To avoid unreliability due to interfering signals from other users of frequency bands in which unlicensed operation is permitted, systems using high frequency spread spectrum techniques for distributing control signals are described in U.S. Pat. Nos. 5,090,024, 5,263,046, 5,278,862 and 5,359,625. The last of these approximates a swept frequency waveform in which successive square waves are formed by the chirps of the sequence, and the frequency of the square waves is varied over at least a portion of the sequence. This technique is suggested for use in the 900+MHz band.
A control system sold under the name Echelon uses microprocessors for control of direct link communication to each of the individual controls of the network over a common channel. This system caters to a wide variety of applications,, and can have as much as a 1 Mbit/sec communication capacity. This system uses a communication protocol which specifies a packet structure, handshake commands to set up a communication and acknowledge a communication, certain error correction and recovery, and retransmission after a time delay if a communication is lost. Transmission is possible over various media, such as twisted pair, radiated RF, infrared, or high frequency signals carried on the power line, between the central source and each of the nodes, except where a relay may be provided to a group of nodes. As a result, installation of such a system is expensive and requires considerable development time. The Echelon system can be used in the license-free 49 MHz band when power is less than 1 watt. Especially if an RF signal is transmitted over the power lines, this system employs a spread spectrum encoding to provide noise immunity.